Producing electrical contact material



Sept. 2, 1947. J. wuLFF O 2,426,659

IRODUCING ELECTRICAL CONTACT MATERIAL Original Filed March 17, 1941SENSITIZING FURNACE CHARGE CORROSION VAT SCREEN PASSIVATIQN vAi'CLASSIFIER DRYER MAGNETH: 7 SEPARATOR -Q SCREENS G INVENTOR.

RADED POWDER .folu; wu jf 2 A T7 RNEY Patented Sept. 2, 1947 PRODUCINGELECTRECAL CONTACT MATERIAL John Wulff, Cambridge, Mass.

Original application March 17, 1941, Serial No.

Divided and this application March 17, 1943, Serial No. 479,436

4 Claims.

This invention relates to producing electrical contact material. Thiapplication i a division of prior application Serial No. 383,875, filedMarch 1'7, 1941, now U. S. Patent 2, 407,862.

In the field of ferrous powder metallurgy there is an increasing demandfor products of improved physical and chemical characteristics, such asincreased tensile strength, corrosion resistance and the like. Suchimprovements are dimcult to attain when utilizing relatively pure ironpowder as a starting material, for any improvement in properties whichis achieved is secured by expensive treatments, such as hot pressing anddiffusion of secondary constituents, such as carbon and the like, intothe iron matrix.

It is well known in the field of fusion metallurgy that the mosteffective and economical method of modifying the physical andphysiochemical characteristics of ferrous base products is to combinepredetermined alloy constituents with the base metal in the melt so asto secure an ultimate alloy of the desired analysis and correspondingphysical characteristics. In the field of powder metallurgy thedesirability of utilizing alloy powders ha been recognized and attemptshave been made to produce such products. suggested methods however havebeen so expensiVe as to preclude commercial production. This highexpense has been largely due to the fact that the production of thealloy has been based on a special treatment of the prefabricated ironpowder, as for example by efiecting diffusion of one or more solid phasealloying constituents into the solid phase iron powder or powdercompact.

The present invention relates to the production of special alloy powdersby utilizing cheap, readily available scrap material containing thedesired alloy constituents and reducing this material to the powder formby utilizing what, for the sake of a term, may be called an inherentweaknes in the starting material. Because of the desirablecharacteristics of stainless steel as a superior metal for theproduction of high strength, corrosion and heat resistant products thismaterial will be chosen as a medium to illustrate the fundamentalconcept of the invention. It will be understood, however, that theinvention may be availed of for the production of powder from anyalloys, whether ferrous or non-ferrous, which exhibit the specialmetallurgical characteristics which are utilized as herein described.

It is known that one of the major shortcomings of austenitic steels ofthe stainless type containing approximately 18 to 25% chromium and 8 to20% nickel and above 02% carbon i their Thesusceptibility tointergranular corrosion when heated to a temperature between about 500C. and 900 C. for an appreciable period of time. In these circumstancesthecarbon which existed in metastable solid solution is rejected in theform of a chromium-rich carbide in the grain boundaries. The steel whichis thus sensitized is susceptible to preferential corrosion by a numberof different corrodents. When subjected to such preferential boundariesbecome weak and the erstwhile massive material i readily crushed toapowder.

The present invention utilizes this intergranular corrosionsusceptibility of such steels to produce an alloy steel powder ofdesirably low carbon content.

Utilizing such a concept it will be appreciated that powders may beproduced utilizing cheap equipment and with simple technique. In ordermore effectively to explain the invention, a diagrammatic illustration,in flow sheet form is shown in the accompanying drawing.

The essential operations involved in the improved process is a heattreatment or sensitization of cheap starting material such as sheet orshot and a preferential disintegration of the boundary areas, preferablyby chemical corrodents, to produce particles corresponding to the grainsize of the material treated.

With these major steps there are employed other steps, such asmechanical disintegration, classification, brightening and/orpassivation,

which imp-roves the product for certain uses. As will be appreciated,the

products produced under the principles of the invention may be widelyutilized. The classified or graded powder may be utilized either aloneor in a blend with iron powder to produce powder metal compacts. As willbe seen more fully hereinafter, chromiumnickel in alloy powders ofextremely low carbon content may readily be produced by the new method.Such powders may be compacted or briquetted with any suitable bondingagents and may be employed as an addition agent to an ironchromium oriron-nickel chromium melt of a relatively high carbon content to securean ultimate alloy of a desirably low carbon content.

The low carbon content of the products producible under the inventionrenders them exceptionally useful for welding rods. The powders may becompacted to the desired shape and coated with suitable fluxing agentsto produce Welding metal of very low carbon analysis.

The iron-chromium powders described herein with peculiar advantage maybe mixed or blended with predetermined percentages of silver powder andthen compacted to produce excellent electrical contact material. Sincean alloy of 18% of chromium and 8% of nickel has the sameelectrochemical potential as silver such contact materials present theadvantages of high corrosion resistance and excellent electricalconductivity.

In carrying out the invention cheap source material may be utilized.Because of its adequate supply, ready availability and low coststainless steel scrap is preferred. This may be utilized in the form ofborings and turnings, shotted material or sheet trim scrap. While notnecessary in operating the process it is preferable, when sheet trim isused, to employ a relatively thin gauge scrap since the use of the thinsection material insures a more rapid and uniform corrosion treatment.It is also desirable to utilize the cold rolled scrap. When hot rolledscrap is employed the scale may be removed in a manner well known tothose skilled in the art after the heat treatment, subsequently to bedescribed, or at any other suitable stage in the process. It is alsodesirable, although not essentially necessary, to classify the scrap tothe extent that stabilized stainless scrap, i. e. columbium and titaniumbearing stainless is culled from the charge. If such preliminaryclassification is not desirable the mixed scrap may be employed sincethe stabilized scrap is readily separable from the sensitized materialduring the course of the treatment. With certain scrap materialsintergranular corrosion may be accelerated by heating in a carburizingatmosphere, as for example in an atmosphere of illuminating gas.

As is well known, the grain size of the alloy which is utilized as asource material will vary depending upon the thermal and mechanicalhistory of the scrap. A typical charge of scrap may consist only of hotrolled material or cold rolled material or a mixture of these two. Thecold rolled material may also be of diiferent degrees of reduction. Theparticle size and particle size distribution required in the ultimatepowder may, if desired, definitely be established in the charge bymaking this up of predetermined mixtures of hot and cold rolled materialwhich latter may itself comprise scrap of different degrees of coldreduction. Where circumstances so dictate the charge may be standardizedas to the eventual particle size by heat treating the mixed scrap priorto the heat sensitizing treatment. This step, as will be appreciated,may be carried out in the furnace employed for the sensitizing treatmentas a preliminary step in the process. As will thus be appreciated theprocess is operable with run-of-mill scrap which varies widely in lengthand cross section as well as with smaller scrap, such as borings,turnings and the like.

The process may be carried out with simple standard equipment andchemicals and with a modicum of labor and control. As shown in thedrawing the scrap to be treated is charged to the furnace I. If desiredprior to such charging the scrap may be treated for the purpose ofcleaning it of hot scale or for any other purpose. To facilitatehandling the scrap it preferably is first pressed into a relativelyloose bale and is charged to the furnace in the form of such units.Where bo-rings and turnings are employed it may be charged to a furnacein a suitable container.

The furnace I may be of any suitable type,

such as a gas or oil fired furnace. Typical annealing furnaces serveeffectively. The only essential requirement of the furnace is that itshall be capable of attaining and holding a temperature in the carbideprecipitation range, 1. e. from about 500 C. to 900 C. or above.

In the furnace I the charge of scrap is heated up to the carbideprecipitation range and held Within that range, preferably with cyclicvariations, for a period of time sufficient to insure completesensitizing of the stock.

The time of the sensitizing treatment, as will be appreciated, willdepend upon the characteristics of the material undergoing treatment. Ifthe charge is comprised largely of stainless of approximately 08% carbonwhich has an average cold reduction of 50% the heat treatment may becontinued 8 or 10 hours or less. With this type of stock the furnace maybe charged and brought up to a temperature of the order of 1050 C. andheld at this temperature for a period of about one-half hour after Whichthe temperature may be dropped to about 800 C'. and cooled slowly from800 C. to 400 C. for a period over about two hours.

In the event that the scrap is less severely cold worked the heatingcycle should be prolonged to about 24 hours more or less. For this typeof charge the furnace preferably is raised to a temperature of betweenabout 400 C. and 760 C.

and held at this temperature for about 12 hours. The temperature is thenpreferably slowly dropped to about 400 C. within the next 12 hourperiod. In each case, as will be appreciated, the material is sensitizedin the carbide precipitation range for a period of time adequate toinsure thorough orientated carbide precipitation. Cyclic beatings arepreferred over the maintenance of a steady heat. Cyclic heatings betweenapproximately 400 C. and 900 C. are advantageous and slow cooling fromthi upper temperature down to 400 C. similarly accelerates the desiredaction.

It will be understood that the. heat treatment or sensitizing will alsobe governed in part by the carbon content. In the preceding examples asteel scrap of about 08% carbon constituted the charge. When the carboncontent is higher the heating cycle may be shortened and when the carboncontent is lower the heating cycle should be commensurately prolonged.For very low carbon scrap, for example, .05% C, the heating time must beincreased tofrom about two to about five times to that given in. thefirst examples. When the carbon content of the scrap is low, i. e. lessthan 06% C, it is preferable to sensitize in a carburizing atmosphere.

After the scrap has been sensitized in the manner described it is thengiven a chemical disintegration treatment. If desired prior to suchchemical disintegration the material may be pickled by any suitableagent to remove oxide. A shown in the drawing such chemicaldisintegration is accomplished by charging the treated scrap to the vat2. This vat may be of any suitable material, such as a ceramic or woodenvat lined with stabilized stainless steel or any other material which isresistant to the action of the corrodent and which does not contaminatethe powdered product. In the vat the sensitized scrap i digested in achemical solution which preferentially attacks or corrodes the grainboundaries of the sensitized metal. For this purpose a modified Strausssolution may be employed. This solution, for example, may comprise 47cc. H2SO4 (sp. g. 1.84) and 13 grams CuSOa5H2O per liter.

The solution preferably is held at an elevated temperature of from about80 C. to 100 C. and the scrap is held in the vat for about 2 to 24hours. In these circumstances the steel is well corroded along the grainboundaries. In the course of this corrosion particles of steel varyingfrom fine powder up to relatively large section detach themselves fromthe scrap sheet stock and fall to the bottom of the tank. When smallermaterial, such as borings and turnings are employed this issubstantially all reduced to a powder in the vat. When sheet stock isused the quantity of the disintegrated detached material may beincreased by occasionally pounding the scrap during the latter stages ofthe chemical corrosion. It will be appreciated that chemical solutionsother than that mentioned may be utilized for this intergranularcorrosion. Thus in lieu of the solution mentioned there may be employedsuch corrodents as dilute sulphuric acid, ammonium sulphate andsulphuric acid, ferric chloride, copper chloride, potassiumhypochlorite, trichlorethylene, hot salt solutions and the like. Asindicated previously, any solution which preferentially attacks thecarbide-rich grains may be utilized.

After the corrosion treatment is completed the corroded material may beremoved from the vat 2 by any suitable means and thoroughly washed inthe container 3. The washed material from which adherent corrodent hasbeen removed may then be passed to the ball mill 4 and milled therein toreduce the larger particles to a powder. It will be appreciated that inlieu of theball mill shown in the drawings any other disintegratingmechanism, such as a hammer, stamp mill or tumbling mill, may beemployed. The time of treatment in the ball mill will, of course, dependupon the type of material treated and the degree to which the sensitizedproduct has been corroded. Reduction in a tumbling mill tends topreserve the powder as single crystals. Other types of milling tend tomake the powder poly-crystalline. As will be appreciated, if thematerial is cold worked during the disintegrating operation it tends tomake the resulting powder magnetic.

After thorough disintegration in the ball mill a charge of powder isdischarged from the mill to the screen 5. The material which has notbeen powdered or pulverized is returned, as shown by line 6, to thecorrosion vat for further treatment. The material passing through thescreen 5, which preferably is less than 150 mesh, is charged to asuitable container 1 in which it is treated with a suitable solution,such as nitric acid which serve to dissolve the copper which plates outfrom the Strauss solution and brighten the powder as well as topassivate the stainless steel particles. For this treatment a commercialnitric acid solution may be employed. Preferably the temperature of theliquid in the vat 1 is maintained between about 54 C. and 60 C.

In some circumstances where the charge of scrap employed is contaminatedwith inorganic materials it may be desirable to treat the product toinsure their removal. For this purpose the material from vat 1 may bewashed and then passed to a suitable classifier 8 which may, for examplebe a Wilfiey table. In this classification treatment extraneousinorganic material is removed. The classified metallic powder may thenbe discharged to the drier 9. This preferably comprises a rotary drierin which the material is dried down at temperatures maintained at 215 C.more or less.

The product which is discharged from the drier comprises metallicparticles of stainless steel intermixed with metal carbides. In order tofractionate this material and to procure a metal powder free from thecarbides the product may be treated in the magnetic separator Hi. Intypical operation the carbides contained in the mass exist in the fines,i. e, the material of minus 325 mesh. When the product discharged fromthe drier is at a temperature of 215 R, which is substantially the Curiepoint of the carbides, magnetic separation is greatly facilitated. Thecarbide material is discharged from the separator at I l and themetallic powder may be passed to the screen system l2 from which powderof difierential particle size is recovered. It will be appreciated thatthe above described process presents an eminently simple method ofproducing metallic powder of predetermined particle size for powdermetallurgy uses. Stainless steel powder so produced is singularly freefrom metal carbides and oxides, these having been removed during thecourse of the treatment. By reason of the freedom from oxides andcarbides the material is extremely plastic and lends itself most readilyto compacting and sintering. In the event that the powder is cold workedto a considerable extent, as for example by reason of a long ballmilling operation, plasticity may be restored by annealing attemperatures not higher than 900 C. for about an hour more or less andthen slowly cooled. Preferably this annealing is effected in dryhydrogen or a vacuum in order to avoid undue oxidation or carburization.

Stainless steel powder has been produced in accordance with thepreceding process with eminently satisfactory results. A chargecomprised of stainless steel sheet trimmings was annealed in asensitizing furnace for a period of 24 hours during about 8 hours ofwhich the product was held within the temperature of the carbideprecipitation range. This product was then corroded for about,24 hoursin the Strauss solution previously described. Upon disintegration andclassification, i. e. the separation of the powder produced in thecorrosion treatment from the larger sections and removal of oxides andthe like an eminently satisfactory product was produced. Upon a screenanalysis this product was found to comprise 30% of powder between and200 mesh, 54% between 200 and 325 mesh and 6% below 325 mesh. Thisparticle size dispersion is thus practically ideal for compacting. Acold compact made up of the powder thus'produced withstood a pressure of80 tons per square inch without cracking. This, as will be appreciated,compares most favorably with typical iron powder compacts which displaya tendency to crack under a pressure of 40 to 50 tons per square inch.

In the preferred method of producing articles from the stainless steelpowder the cold pressed compact is preferably sintered in a hydrogenatmosphere. When sintering at a temperature between 900 C. and 1250 C.for minutes, products have been produced which possess a tensilestrength in excess of 60,000 pounds per square inch with a reduction ofmore than 10% in area. It will be appreciated that with these improvedphysical characteristics combined with the excellent chemicalcharacteristics of stainless steel, sintered products possess a Verywide permissive field of use. The stainless steel pow'der so producedmay, of course, be mixed or blended with other metal powder, such asiron powder, or any desired type silver powder, nickel powder and thelike so as to produce products of variant physical,

7 physicochemical and/or electrical characteristics.

As will be appreciated, the physicals of the compacts produced under thepresent invention are such as to permit subsequent processing so as tomore effectively conform the compacts to an intended use. Thesecompacts, for example may be forged and hot rolled to produce articlesof physical characteristics comparable to products produced by typicalfusion metallurgy methods.

It will be understood that the process of producing the stainless steelpowder herein described and as illustrated by the flow sheet issusceptible of many modifications. As previously explained, the processis available for use with different types of scrap, i. e. hot and coldrolled scrap. Where hot rolled scrap is used it is desirable to picklethe material at some suitable stage to remove oxides. Where oxideremoval is complete prior to passivation or drying physicalclassification or separation as illustrated at stage 8 may be dispensedwith. Obviously in lieu of the particular apparatus shown othersimilarly functioning apparatus may be employed. pointed out, thecarbides occurring in the material are largely included in the fines theclassification in a mechanical classifier may be sunlciently effectiveso as topreclude the necessity of magnetic separation. Again, as will beappre= ciated, certain of the separate stages described herein may becombined. Again the sensitizing treatment may be carried out in a rotarykiln in which a corrosive gaseous atmosphere is maintained.

The stainless steel powder produced as described herein may beclassified as desired so as to give any predetermined particle sizedistribution. For high pressure pressing, i. e. over 30 tons per squareinch, a powder is preferred containing about 66 of between 100 and 200mesh material, Mi more or less of between 200 and 325 mesh material,about 17% of between 200 and 325 mesh material and 16% below 325 mesh.For lower pressure pressing a preferred product comprises about 42% ofmaterial between 100 and 200 mesh, about 42% of material between 200 and325 mesh and about 16% of material below 325 mesh. As is understood bythose skilled in the art, in making up the powder mixture for compactlubricants, such as graphite, may be introduced. Similarly silver powdermay be utilized for its lubrication. As noted above, the particularanalysis of the powder employed for the compacting will be determinedupon the desired use. Thus for bearing material the stainless steelpowder may be admixed with copper or silver phosphide or low meltingsilver solders so as to improve the bearing surface. These and othermodifications and ramifications will occur to those skilled in the artof utilizing the improved product.

It is clearly to be understood, as explained earlier that the presentinvention comprehends the broad concept of producing metal powders fromcheap source material by invoking and utilizing an inherentcharacteristic of such source material for powder production. Stainlesssteel has been chosen herein as illustrative of a classical material ofthis type. The invention, however, comprehends the production of metalpowders or metal alloy powders from any source material which, by reasonof a heat treatment, is embrittled and/or rendered corrodible in theboundary areas, The invention is, therefore, appl-icable to any alloywhether of the solid solution Again since, as

type or notwhich through heat and/or mechanical treatment develops grainboundary weakness which thus permits disintegration of such boundaryareas. For certain products like stainless steel separation into powderis facilitated by chemically corroding this differential boundary phase.In certain other alloys a liquid corrodent need not be employed andparticularly where the grain boundary phase is of a brittle character.In such circumstances powdering of the material after the heatsensitizing treatment may be effected by any suitable type oftrituration or grinding. Whenever desired this grinding may be in aliquid menstruum which serves as a lubricant and/or as a protectivematerial to preclude oxidation and the like. Typical of such productswhich may be treated in accordance with the invention arealuminum-magnesium alloys, aluminum silicon alloys and similarnon-ferrous products which upon heat sensitizing or mechanical workingdevelop an intergranular embrittled phase which may readily bemechanically reduced or corroded so as to produce a powder of a particlesize corresponding to the grain size of the matrix after the heattreatment. Thus magnesium aluminum alloys containing up to 10% ofaluminum may be employed. For example, brass may be heat treated andcold worked after which the product may be milled in the presence of asuitable corrodent, such as a mercurous nitrate solution of suitablestrength. Again Armco iron may be heated to about 1315 C., slowly'cooledand then milled in the presence of ammonium nitrate to produce apowdered product. Other alloys amenable to the treatment to producemetallic powders are: magnesium-bismuth alloys containing up to 10%bismuth; magnesium-copper alloys containing up to 5% copper;magnesium-lead alloys containing up to 10% lead; magnesimn-zinc alloyscontaining up to 8.4% zinc; and magnesium-silicon containing up to about1.4% silicon. The above mentioned alloys, of course, are not exclusiveor extensive but are merely illustrative of the general typecomprehended herein.

While preferred embodiments of the invention have been described it isto be understood that these are given didactically to illustrate theunderlying principles involved and not as limiting the invention to thechosen examples.

I claim:

1. A method of improving the pressing characteristics of high chromiumsteel powders comprising adding thereto a sunicient amount of silverpowder to impart lubricity to the steel powder.

2. A method of producing electrical contact material which comprises,admixing stainless steel powder of predetermined particle sizedistribution and silver powder, cold pressing the mix in the desiredshape and sintering the resulting compact below the melting point ofsilver.

3. A method of producing electrical contact material which comprises,admixing stainless steel powder of predetermined particle sizedistribution and silver powder, cold pressing the mix in the desiredshape and sintering the resulting compact below the melting point ofsilver in a reducing atmosphere.

4. A method of producing electrical contact material which comprises,admixing stainless steel powder and a predetermined amount of silverpowder, thestainless steel powder having a particle size distribution ofabout 66% between and 200,mesh, about 17% between 200'and 325 9 mesh andabout 17% below 325 mesh, cold pressing the mix in the desired shape andsintering the resulting compact below the melting point of silver.

JOHN WULFF.

REFERENCES CITED The following references are of record in the file ofthis patent:

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